1. Field of the Invention
The present invention is related to a method and device for driving a two-phase brushless motor, and more particularly, to a method and device for detecting a position of a rotator of the motor according to an output voltage of a stator coil.
2. Description of the Prior Art
With advances in electronic circuit technology, heat dissipation problems are increasingly critical for electronic devices. To enhance heat dissipation efficiency, heat dissipation fans are embedded in the electronic devices, implying even more stringent specification requirements for weight, noise, and cost of the heat dissipation fan. Compared to a conventional direct current (DC) brush motor, a two-phase brushless motor has advantages of light weight, rapid acceleration, and low noise, and therefore is widely employed in personal computer components, such as a central processing unit (CPU), a graphics card, and a power supply.
Please refer to FIG. 1, which is a schematic diagram of a two-phase brushless motor system 10 of the prior art. The two-phase brushless motor system 10 includes a rotator 100, a stator 110, a control circuit 120, a driving unit 130 and a Hall sensor 132. The rotator 100 includes four magnetic poles (two with north (N) magnetism and the other two with south (S) magnetism). The stator 110 includes a first coil 112 and a second coil 114 for varying magnetic field its vicinity through electromagnetic induction to drive the rotator 100. The Hall sensor 132 is generally installed on the stator 110, and is utilized for sensing magnetic field variances induced by the rotating rotator 100, i.e. determining a current position of the Hall sensor 132 corresponding to N or S magnetism, to generate a sensing signal SEN carrying positional information of the rotator 100 sent to the control circuit 120. The control circuit 120 generates commutation signals CMU[1], CMU[2] sent to the driving unit 130. The driving unit 130 includes a first switch transistor 134 and a second switch transistor 136 respectively utilized for controlling currents passing through the first coil 112 and the second coil 114 according to the corresponding commutation signals CMU[1], CMU[2] to ensure that the rotator 100 can rotate continuously.
In short, the two-phase brushless motor system 10 detects the position of the rotator 100 through the Hall sensor 132 to determine commutation time points between the first coil 112 and the second coil 114, and accordingly switches the conducting coil thereof through the driving unit 130 at the commutation time points. As a result, by periodically detecting the position of the rotator 100 and switching the conducting coil, the two-phase brushless motor system 10 can ensure that the rotator 100 rotates without interruption. However, due to limited sensitivity, the Hall sensor 132 cannot correctly detect the position of the rotator 100 when the magnetic variances are insignificant, e.g. during a period in which the motor just begins to rotate. In such a situation, the control circuit 120 cannot determine the next commutation time point, leading to undetermined operation statuses of the first coil 112 and second coil 114. Once the magnetic field provided by the stator 110 stops alternating, the rotator 100 stops rotating. In addition, installation of the costly Hall sensor 132 is disadvantageous for minimizing volume and cost of the two-phase brushless motor system 10, especially when the two-phase brushless motor system 10 has to be embedded within the electronic device.
Therefore, developing a sensing method capable of replacing functions of the Hall sensor to decrease manufacturing cost of the two-phase brushless motor has been a major focus of the industry.